using System.Security.Cryptography;
using System.Text;

namespace DigitalData.Core.Security
{
    public class RSAFactory
    {
        private static readonly Lazy<RSAFactory> LazyInstance = new(() => new());

        public static RSAFactory Static => LazyInstance.Value;

        public static readonly string DefaultEncryptedPrivateKeyFileTag = "enc-private";

        public static readonly string DefaultPrivateKeyFileTag = "private";

        public static readonly string DefaultPublicKeyFileTag = "public";

        public static readonly IEnumerable<string> KeyFileTags = new string[] { DefaultEncryptedPrivateKeyFileTag, DefaultPrivateKeyFileTag, DefaultPublicKeyFileTag };

        public static readonly string PEMFileExtension = ".pem";

        private static readonly Lazy<IEnumerable<string>> LazyLowerFileTags = new(() => KeyFileTags.Select(tag => tag.ToLower()));

        public static readonly string DefaultRSAKeyNameSeparator = "-_-";

        //TODO: make the validation using regex
        public static string DefaultRSAKeyNameFormatter(string issuer, string audience, bool isPrivate = true, Version? passwordVersion = null, string? separator = null)
        {
            separator ??= DefaultRSAKeyNameSeparator;

            void ValidateForbidden(string value, string paramName)
            {
                if (Path.GetInvalidFileNameChars().Any(value.Contains) || LazyLowerFileTags.Value.Any(tag => value.ToLower().Contains(tag)))
                    throw new ArgumentException($"RSA decryptor key name creation is forbidden. The {paramName} contains forbidden characters that are not allowed in file naming.", paramName);
            }

            static void ValidateSeparator(string value, string paramName, string separator)
            {
                if (value.Contains(separator))
                    throw new ArgumentException($"RSA decryptor key name creation is forbidden. The {paramName} contains separator characters ({separator}) that are not allowed in file naming.", paramName);
            }

            ValidateForbidden(issuer, nameof(issuer));
            ValidateForbidden(audience, nameof(audience));
            ValidateForbidden(separator, nameof(separator));

            ValidateSeparator(issuer, nameof(issuer), separator);
            ValidateSeparator(audience, nameof(audience), separator);

            var sb = new StringBuilder(issuer.Length + audience.Length + separator.Length * 2 + 20);
            sb.Append(issuer).Append(separator).Append(audience).Append(separator);

            if (passwordVersion is null && isPrivate)
                sb.Append(DefaultPrivateKeyFileTag);
            else if (isPrivate)
                sb.Append(DefaultEncryptedPrivateKeyFileTag).Append(separator).Append(passwordVersion);
            else if (passwordVersion is null)
                sb.Append(DefaultPublicKeyFileTag);
            else
                sb.Append(DefaultPublicKeyFileTag).Append(separator).Append(passwordVersion);

            var rsaKey = sb.Append(PEMFileExtension).ToString();

            return rsaKey;
        }

        public int KeySizeInBits { get; init; } = 2048;

        public string PbePassword { private get; init; } = Secrets.PBE_PASSWORD;

        public PbeEncryptionAlgorithm PbeEncryptionAlgorithm { get; init; } = PbeEncryptionAlgorithm.Aes256Cbc;

        public HashAlgorithmName PbeHashAlgorithmName { get; init; } = HashAlgorithmName.SHA256;

        public int PbeIterationCount { get; init; } = 100_000;

        private readonly Lazy<PbeParameters> _lazyPbeParameters;

        public PbeParameters PbeParameters => _lazyPbeParameters.Value;

        public string EncryptedPrivateKeyPemLabel { get; init; } = "ENCRYPTED PRIVATE KEY";

        internal RSAFactory()
        {
            _lazyPbeParameters = new(() => new PbeParameters(PbeEncryptionAlgorithm, PbeHashAlgorithmName, PbeIterationCount));
        }

        public string CreateRSAPrivateKeyPem(int? keySizeInBits = null)
            => RSA.Create(keySizeInBits ?? KeySizeInBits).ExportRSAPrivateKeyPem();

        public string CreateEncryptedPrivateKeyPem(
            int? keySizeInBits = null,
            string? password = null,
            PbeEncryptionAlgorithm? pbeEncryptionAlgorithm = null,
            HashAlgorithmName? hashAlgorithmName = null,
            int? iterationCount = null)
        {
            password ??= PbePassword;

            var pbeParameters = (pbeEncryptionAlgorithm is null && hashAlgorithmName is null && iterationCount is null)
                ? new PbeParameters(
                    pbeEncryptionAlgorithm ?? PbeEncryptionAlgorithm,
                    hashAlgorithmName ?? PbeHashAlgorithmName,
                    iterationCount ?? PbeIterationCount)
                : PbeParameters;
            
            var encryptedPrivateKey = RSA.Create(keySizeInBits ?? KeySizeInBits).ExportEncryptedPkcs8PrivateKey(password.AsSpan(), pbeParameters);

            var pemChars = PemEncoding.Write(EncryptedPrivateKeyPemLabel, encryptedPrivateKey);

            return new string(pemChars);
        }
    }
}